1. Field of the Invention
The present invention generally relates to an optical disk storage device, such as an optical disk device or a magneto-optic disk device, and more particularly to an optical disk storage device in which light is projected onto a recording film via a protection film formed on the recording film.
2. Description of the Prior Art
An optical disk has a recording film formed on a substrate, and a protection film which covers the recording film. It will be noted that the term "optical disks" includes magneto-optic disks. Recently, an attempt to reduce the thickness of optical disk storage devices that use the optical disks described above has been made. For this purpose, it is necessary to diminish the dimensions of optical parts, such as objective lenses.
FIG. 1 illustrates a conventional optical disk storage device 1. A laser beam 2 is projected onto an optical disk 3 at the side of a base 4 thereof. A recording film 5 is formed on the base 4, and a protection film 6 is formed on the recording film 5. An optical system includes a semiconductor laser 7, a beam splitter 8, an objective lens Q, and a photodetector 10. The laser beam 2 emitted from the semiconductor laser 7 passes through the beam splitter 8 and is focused on the recording film 5 by the objective lens 2. The laser beam 2 is reflected by the recording film 5, and passes through the objective lens 9. Then, the reflected laser beam is reflected by the beam splitter 8, and is protected onto the photodetector 10. The photodetector 10 converts the received light into a corresponding electric signal. In this manner, information recorded on the optical disk 3 can be reproduced.
The laser beam 2 through the objective lens g is refracted at the surface of the base 4, and passes therethrough. Normally, the base 4 is thick. For example, the thickness t1 of the base is 1.2 mm. The size d1 of a light spot formed on the surface of the base 4 needs to be large enough to focus the laser beam 2 on the surface of the recording film 5. Hence, it is necessary to use a large size of objective lens 9 and a large size of beam splitter 8. As a result, with the structure shown in FIG. 1, it is very difficult to reduce the dimensions of the optical parts and hence produce thin optical disk storage devices.
In order to eliminate the above-described disadvantages, an improved optical disk storage device 20 as shown in FIG. 2 can be made. A laser beam 21 is projected onto the optical disk 3 from the side of the protection film 6. The laser beam 21 emitted from the semiconductor laser 7 passes through a beam splitter 23, and is converged by an objective lens 22. The converged laser beam 21 passes through the protection film 6, and is projected onto the recording film 5. Then, the laser beam 21 is reflected by the surface of the recording film 5, and is projected onto the photodetector 10 via the objective lens 22 and the beam splitter 23. The laser beam 21 is refracted at the surface of the protection film 6.
Normally, the thickness t2 of the protection film 6 is approximately 30 .mu.m, and is much less than the thickness t1 of the base 4. Hence, the size d2 of the light spot on the surface of the protection film 6 is far less than the spot size d1 shown in FIG. 1. As a result, it is possible to use an objective lens 22 of a smaller size and a beam splitter 23 of a smaller size. Hence, the optical disk storage device 20 can be made more compact than the optical disk storage device 1. In addition, a large tolerance for the inclination of the optical disk 3 can be obtained because the laser beam 22 is refracted at the surface of the thin protection film 6 and then travels a very short optical distance to reach the recording film 5.
The inventors of the present invention found a disadvantage of the optical disk storage device 20 shown in FIG. 2 through experiments. The inventors studied the relationship between the position of the photodetector 10 and the reproduced information signal output from the photodetector 10. When the photodetector 10 is located at an appropriate position, the photodetector 10 generates an output signal 30 shown in FIG. 3B. The output signal 30 shown in FIG. 3B has an approximately constant envelope. When the photodetector 10 is located at an inappropriate position, the photodetector 10 generates an output signal 31 shown in FIG. 3A. The envelope of the output signal 31 varies due to interference between the laser beam reflected by the surface of the recording film 5 and the laser beam reflected by the surface of the protection film 6.
In the prior art, there is no consideration given to the above problem.